I agree that "The Legacy" is the better, more appropriate song. It's flamboyant and self-serving, instead of the more sincere "Because Of Her". But "The Legacy" is also problematic. It's a star spot for Oscar, after Lily has just had the show-stopping "Babette". But "The Legacy" is a list song that pretty much plays its hand in the first verse, and then has two more verses to go, where it mostly repeats itself. List songs need to keep topping themselves in wit, invention or sheer absurdity, the way Irving Berlin or Cole Porter were often able to do.
I'm sure it was meant to be a showstopper for Oscar, and one can just imagine Adolph Green selling the hell out of it in backers auditions. But it didn't create a major impression in the original production, even with a game John Cullum delightfully trilling his consonants. Over at All That Chat, AlanScott, who saw the original production multiple times said in a recent thread that he only saw the number really land once - at closing, when Cullum gave it absolutely everything he had in him. AlanScott theorized that perhaps the song needed that type of go-for-broke commitment all along, but in practice, in a farce that's as much of a heavy sing as On The Twentieth Century, few actors would be able to sustain that level of performance for eight shows a week.
Begin at the beginning and go on till you come to the end: then stop.